Oculopharyngeal muscular dystrophy (OPMD) is a rare autosomal dominant disease of late onset for which no cure exists. It is characterized primarily by eyelid drooping and difficulties in swallowing with some weakness in proximal limb muscles. Although mutations in the ubiquitously expressed PABPN1, an mRNA binding protein, cause OPMD, much is still unknown regarding the mechanism by which mutant PABPN1 leads to muscle-specific pathology. In autosomal dominant OPMD one mutant allele of PABPN1 replaces one normal allele of PABPN1. Thus, pathology could be due to the loss of one normal allele, the gain of a mutant allele or the combination of both events. Due to the fact that PABPN1 appears to play an essential role in RNA metabolism, any impairment of its function should, in theory, affect numerous cell and tissue types, but the intrinsic characteristics of skeletal muscle may make this tissue more vulnerable to the effects of mutant PABPN1. Indeed, our preliminary data reveal that muscle tissue shows significantly lower levels of PABPN1 protein as compared to unaffected tissues. We hypothesize that the lower amount of PABPN1 in skeletal muscle could sensitize this tissue to the deleterious effects of mutant PABPN1. The overall goal of this proposal is to examine how both mutant PABPN1 and decreased functional levels of wild type PABPN1 impact RNA biogenesis and myogenesis. Thus, we will analyze the consequences of expressing mutant PABPN1 in muscle cells (Aims 1 and 2). We will exploit a PABPN1 knockout mouse to determine whether a decrease in PABPN1 is sufficient to cause muscle pathology or sensitize cells to the expression of mutant PABPN1 (Aim 2). Finally, we will identify RNAs that are altered in response to decreased functional levels of PABPN1 in muscle cells and consider key muscle functions for these putative PABPN1 targets (Aim 3). Importantly, the Specific Aims are designed to understand the muscle-specific role of PABPN1, which is critical for understanding the pathogenesis of OPMD. The knowledge gained from our studies is likely to afford new therapeutic strategies that target the appropriate molecular pathways altered in the muscles of OPMD patients.